pH-dependent Charge
With many protons, many protonations.
Definitions:
P “Proton” = H+ ion (quicker to say).
P “Protonation” = to stick protons on “stuff”. The
“sticking” usually involves forming a covalent bond
between the H+ ion and the stickee, or (in the case
of clays) at least a bond with a good deal of
“covalent character”.
P “Stuff”:
< Dissolved organic or inorganic molecules or ions.
< Chemically suitable “spots” on surfaces. On clays, edges
of layers have lots of “suitable spots”. On kaolinite, and
the sesquioxides of Fe and Al, octahedral sheets at layer
surfaces have lots of “suitable spots”, as will be shown,
shortly.
P Deprotonation = yanking protons off stuff.
Some spots like protons; some
hate them.
You can try all day long, but you will have a hard time yanking
the proton off an alcohol molecule.
The protons in a glass of booze are so reluctant to come off that
you would have to increase the pH of your drink to about 18 before
half of the ethanol molecules in your glass look like the (ethanolate)
anion on the right
Alas, nobody has ever created an aqueous solution with a pH much
above 16, so the ethanolate anion must be considered to be a “rare
species”.
Conversely, it’s easier than sin in New Orleans to protonate an
ethanolate anion.
“Stuff” in soil
Well humified, colloidal organic matter particles have lots of attractive
spots suitable for protonation. One common and fetching example is
the “carboxylate anion”, which is often written in text as R-CO2!, but
which is better explained in pictures.
The proton on a carboxylate group comes off nice and easy.
R-COOH X R-COO! + H+.
Depending on the local chemical environment, most carboxylate
groups are half-protonated at about pH 4.5.
Thus, you need a lot more protons in solution to protonate carboxylate
anions than to protonate ethanolate anions.
Self-image Problems
Back when you were eight, a nice third-grade teacher told you
that “like charges repel”. This powerful truth applies even to a
single charge, e.g., of !1. It hates itself and wants to get away
from its own stink by spreading out over many atoms.
Trapped like a rat! Concentrated around a single O atom
A much happier negative
charge schmeared over
three atoms. Whew!
Room to breathe.
Fair play. When no obvious reason explains why one of two identical
atoms gets the double bond and the other gets stuck with the negative
charge, the two atoms “split the difference”. Both get a bond and a
half and a partial negative charge. Chemists call this “resonance”.
Organic chemical shorthand
The rules:
Carbon makes four bonds.
P Nitrogen makes three bonds, unless noted otherwise.
P Oxygen makes two.
P Hydrogen makes one.
P
The conventions:
Straight lines represent covalent bonds.
P At an unlabeled angle and at the end of a line, a C atom is
implied.
P If you want a different atom, label it.
P Mentally add implied H atoms ‘til everybody is happy.
P
Positive charge on these colloids (last column) is probably measured at
a pH of 3.5, since that pH nearly maximizes positive charge without
being so acidic that the clays begin to disintegrate.
One more ugly fact about old clays
(i.e., about kaolinite and hydrous oxides of Fe and Al)
They bind phosphate tighter than derivatives traders
cling to their tax breaks.
Save your breath and don’t try to tell a phosphate ion
that it has a negative charge and should repel
negatively charged clay particles. It won’t listen and
will stick anyway.
Phosphate anions are illiterate and never went to third grade.